1. Field of the Invention
The invention concerns a flexible carrier web having on at least a portion thereof a silicone layer and a method by which a layer of silicone can be applied to a flexible carrier web. The invention also concerns pressure-sensitive adhesive tapes that employ such flexible carrier webs as backings.
2. Description of the Related Art
It has long been recognized that silicone layers afford excellent release to layers of pressure-sensitive adhesive tapes, however release values provided by silicone layers tend to be undesirably low for many uses, e.g., about 4-60 g/cm of width. In an attempt to control the release properties, it has been suggested that the silicone be applied only to a portion of the desired substrate.
In U.S. Pat. No. 4,171,397 (Morrow), the release properties are regulated by first coating a sheet substrate with a fluorochemical polymer consisting of a perfluoroalkylsulfonamide lower alkyl acrylate and the reaction product of perfluoroalkyl-sulfonamidolalkanol, toluene diisocyanate, and hydroxypropylmethacrylate. The fluorochemical must have surface energy properties such that the contact angle of a drop of ethanol on a smooth surface thereof is at least 30.degree.. Silicone-forming reactants are then overcoated in a manner providing a patterned silicone layer to attain release values determined by the relative exposed amounts of the fluorochemical and silicone.
In U.S. Pat. No. 3,503,782 (Ayres), a desired release value was obtained by first applying a full coating of a silicone release agent followed by a partial coating of a different silicone, preferably in the form of a pattern.
German Offenlegungsschrift P 37 27 078 (Skurnia et al.), published Feb. 23, 1989, discloses that, while pattern release coatings have been discussed for 30 years, problems associated with processing and use have not been resolved. This reference uses a method wherein two layer screen-printed silicone release coatings are made on a substrate by priming with a commercially available primer. Specifically disclosed primers include isocyanate-linked polyvinyl acetate, and solutions of chromosterate complexes mixed with polyvinyl alcohol. The silicone is then overcoated to cover from 20% to 80% of the surface.
However, because these methods require the complete coating with one primer or release agent which is then overcoated, several processing steps are required, and there is a significant amount of waste.
PCT Appln. Publ. No. WO 90/07560, published Jul. 12, 1990 (Jung et al.), discloses release values in excess of 100 gm per centimeter of width from patterned silicone release coating. The release coating is prepared by engraving an outer roll surface of a roll-coater with a pattern corresponding to the desired silicone pattern, and then coating a substrate with such roll to obtain a definite geometric pattern. However, this method is expensive as it requires a custom-made roll-coater for each pattern desired.
Another use of patterned silicone layers is to confer repositionability onto a tacky adhesive tape. In U.S. Pat. No. 3,554,835 (Morgan), a pattern of silicone dots was applied to the face of a pressure-sensitive adhesive tape. The dots allow a sheet of the tape to be slid over a substrate until pressure is applied to force the adhesive into contact with the substrate.
Patterned silicone layers are produced on printing plates, e.g., "Toray Waterless Plate", available from Toray Industries, and the like. Such a printing plate is a flexible sheet of aluminum bearing a layer of photosensitive material covered with a layer of silicone rubber. Upon exposure to light through a half-tone screen, the photosensitive material of a positive-acting plate binds firmly to the silicone rubber in areas where the light strikes. In the unexposed regions, the silicone can be brushed off, leaving the silicone layer in the predetermined pattern provided by the light exposure. Methods of making a printing plate based on photoimageable silicone are described in U.S. Pat. No. 3,511,178 (Curtin) and in U.S. Pat. No. 4,985,342 (Muramoto et al.).
In U.S. Pat. No. 4,930,266 (Calhoun et al.), a Toray printing plate having a pattern of silicone dots was wrapped on a cylinder and rotated through a fluidized bed of abrasive granules. The granules were attracted to the silicone dots. Upon moving a binder-carrying backing in synchronism with the rotating printing plate, the granules were picked up by and became embedded into the binder in the pattern of the printing plate. By thus individually positioning abrasive granules in a predetermined pattern, abrasive sheeting can produce fine finishes at surprisingly high cutting rates.
Another use of a Toray printing plate is disclosed in EPO Pat. Appln. Publ. No. 0,330,452, published Aug. 30, 1989 (Calhoun et al.). Electrically conductive particles are attracted to silicone dots and then individually transferred to an adhesive layer to make the adhesive layer electrically conductive orthogonally to its face while the adhesive layer remains electrically insulative laterally.
A method has now been discovered whereby single-layer patterned silicone release coatings can be made using conventional coating apparatus.